The present invention relates to a filter, particularly a laminated dielectric filter, which mainly is used in high-frequency radio equipment such as portable telephones.
Recently, with reduction in size of communication equipment, laminated dielectric filters effective for size reduction are used commonly as high-frequency filters. One example of conventional laminated dielectric filters is described with reference to drawings as follows.
FIG. 4 shows an exploded perspective view of a conventional laminated dielectric filter. The conventional laminated dielectric filter shown in FIG. 4 includes dielectric layers 301, shield electrodes 302a and 302b, resonator electrodes 303a, 303b, and 303c, capacitor electrodes 304a, 304b, 305a, 305b, 307a, 307b, and 307c, and side electrodes 308a, 308b, 308c, 308d, 309a, and 309b. In the dielectric layers 301, the shield electrode 302a, the resonator electrodes 303a, 303b, and 303c, the capacitor electrodes 304a, 304b, 305a, 305b, 307a, 307b, and 307c, and the shield electrode 302b are positioned sequentially. In addition, the side electrodes 308a and 308b on the left and right side faces of the dielectric body connect the shield electrodes 302a and 302b to form ground terminals. The side electrode 308c on the back face of the dielectric body connects the shield electrodes 302a and 302b and a common short-circuit end of the resonator electrodes 303a, 303b, and 303c to form a ground terminal. The side electrodes 308d on the front face of the dielectric body connect the capacitor electrodes 307a, 307b, and 307c corresponding to the open ends of the resonator electrodes 303a, 303b, and 303c, respectively. The side electrodes 309a and 309b on the left and right side faces of the dielectric body are connected to the capacitor electrodes 304a and 304b to form input/output terminals.
The structural view of the laminated dielectric filter thus configured is shown in FIGS. 5A and 5B. FIG. 5A is its left side view and FIG. 5B its front view. FIGS. 5A and 5B also show schematic capacitors formed between electrodes formed on an upper surface of a dielectric layer and electrodes formed on an upper surface of another dielectric layer, which oppose each other, respectively.
An equivalent circuit of the conventional laminated dielectric filter shown in FIGS. 4, 5A and 5B can be illustrated as shown in FIG. 6. The resonator electrodes 303a, 303b, and 303c form front end short-circuit xc2xc wavelength resonators R303a, R303b, and R303c as shown in FIG. 6. The open ends of the resonators R303a, R303b, and R303c are connected to the ground terminals via the loading capacitor elements C307a, C307b, and C307c, respectively. The open ends of the resonators R303a and R303b are connected to each other via an inter-stage coupling capacitor element C305a and the open ends of the resonators R303b and R303c via an inter-stage coupling capacitor element C305b. Furthermore, the resonators R303a and R303c on the outer sides are connected to the input/output terminals via input/output coupling capacitor elements C304a and C304b, respectively.
Therefore, the laminated dielectric filter shown in FIG. 4 functions as a bandpass filter with the one ends of the capacitor elements C304a and C304b serving as the input/output ends. In addition, two attenuation poles are formed by a parallel resonance circuit formed of the inter-stage coupling capacitors C305a and C305b and magnetic-field couplings 401a and 401b occurring between the resonators R303a and R303b and between the resonators R303b and R303c. The frequencies of the attenuation poles depend on inter-stage coupling capacitance and the magnitude of the magnetic-field couplings, i.e. resonant gaps.
In the configuration as described above, however, the resonators R303a and R303c on the both sides bypass the resonator R303b positioned at the center to be coupled directly to each other by a magnetic-field coupling as indicated with the numeral 401c. Therefore, frequency characteristics of the two attenuation poles vary and thus the characteristics as designed cannot be obtained. The magnetic-field coupling 401c is determined uniquely when the magnetic-field couplings 401a and 401b are determined, i.e. when the resonant gaps are determined. Consequently, the two attenuation poles cannot be controlled freely while consideration is given to the magnetic-field coupling 401c. 
The present invention is intended to provide a filter, particularly a laminated dielectric filter, allowing attenuation poles outside a passband to be controlled freely.
In one embodiment, a filter of the present invention includes a plurality of resonators coupled to one another by electromagnetic-field coupling. In the embodiment, non-adjacent resonators are electrically coupled to each other with a series circuit formed of a capacitor and a transmission line.
According to the filter of this embodiment, the capacitor formed between the non-adjacent resonators is regulated without being affected by the magnetic-field bypass coupling between the non-adjacent resonators. Thus, attenuation poles outside a passband can be controlled freely.
In the above-mentioned filter, it is preferred to electrically couple adjacent resonators to each other with a series circuit of a capacitor and a transmission line.
According to this configuration, it is possible to control at least two attenuation poles of a parallel resonance circuit formed by the electromagnetic coupling and capacitive coupling between adjacent resonators.
In the above-mentioned filter, it is preferable that the plurality of resonators and the transmission line are formed inside a dielectric body.
According to this configuration, the capacitor as a component of the filter can be formed easily by using the plurality of resonators and the transmission line as electrodes.
In another embodiment, a dielectric filter of the present invention includes a plurality of shield electrodes formed on outer faces of a dielectric body, resonator electrodes formed of at least three front end short-circuit xc2xc wavelength transmission lines, a plurality of first transmission line electrodes, each of which has portions opposing respective portions of two adjacent resonator electrodes included in the resonator electrodes, and second transmission line electrodes having portions opposing the plurality of first transmission electrodes, respectively. The resonator electrodes, the first transmission line electrodes, and the second transmission line electrodes are formed between the plurality of shield electrodes.
In some embodiments, inter-stage coupling capacitors are formed between adjacent resonator electrodes and the first transmission line electrodes opposing them, and bypass capacitors are formed between the first transmission line electrodes and the second transmission line electrodes opposing them. Due to the bypass circuit formed of a series circuit including the bypass capacitors and the second transmission line electrodes, the attenuation poles outside the passband can be controlled freely by the adjustment of capacitance of the inter-stage coupling capacitors without being affected by a magnetic-field bypass coupling between non-adjacent resonator electrodes. Thus, a capacitive coupling type bandpass filter having the above-mentioned effect of controlling the attenuation freely can be obtained.
In the dielectric filter, it is preferable that the plurality of shield electrodes are connected to one another, and then are grounded.
According to this configuration, between the shield electrodes thus grounded, filter components can be positioned. Therefore, without being affected by an external electromagnetic field, desired filter characteristics can be obtained as designed.
In another embodiment, a laminated dielectric filter of the present invention has the following configuration. A first dielectric layer is laminated above a first shield electrode. On the upper surface of the first dielectric layer, resonator electrodes formed of at least three front end short-circuit xc2xc wavelength transmission lines are formed. Above the resonator electrodes, a second dielectric layer is laminated. On the upper surface of the second dielectric layer, a plurality of inter-stage coupling capacitor electrodes are formed. Each of the inter-stage coupling capacitor electrodes is formed of a transmission line having portions opposing respective portions of two adjacent resonator electrodes included in the resonator electrodes. Above the inter-stage coupling capacitor electrodes, a third dielectric layer is laminated. On the upper surface of the third dielectric layer, bypass electrodes are formed. The bypass electrodes are formed of transmission lines having portions opposing the plurality of inter-stage coupling capacitor electrodes, respectively. Above the bypass electrodes, a fourth dielectric layer is laminated. On the upper surface of the fourth dielectric layer, a second shield electrode is positioned.
In some embodiments, inter-stage coupling capacitors are formed between adjacent resonator electrodes on the first dielectric layer and the inter-stage coupling capacitor electrodes on the second dielectric layer opposing the adjacent resonator electrodes. Bypass capacitors are formed between the inter-stage coupling capacitor electrodes on the second dielectric layer and the bypass electrodes on the third dielectric layer opposing them. Due to the bypass circuit of a series circuit including the bypass capacitors and the bypass electrodes, attenuation poles outside a passband can be controlled freely by the adjustment of capacitance of the inter-stage coupling capacitors without being affected by a magnetic-field bypass coupling between non-adjacent resonator electrodes. Thus, a capacitive coupling type bandpass filter having the above-mentioned effect of controlling the attenuation poles freely can be obtained.
In the above-mentioned laminated dielectric filter of the present invention, it is preferable that the first shield electrode is provided on the upper surface of a fifth dielectric layer.
In the above-mentioned laminated dielectric filter of the present invention, it is preferred to laminate a sixth dielectric layer above the second shield electrode.
According to this configuration, the sixth dielectric layer can protect the second shield electrode. In addition, it also is possible to form the same resonator electrodes as those on the first dielectric layer on the upper surface of the six dielectric layer and further laminate the same dielectric layers as the second and third dielectric layers, on the upper surfaces of which the inter-stage coupling capacitor electrodes and the bypass electrodes are formed, respectively, thus obtaining filters separated by the second shield electrode from each other.
In the above-mentioned laminated dielectric filter of the present invention, it is preferable that the first and second shield electrodes are connected to each other and then are grounded.
According to this configuration, filter components can be positioned between the first and second shield electrodes that are grounded. Therefore, desired filter characteristics can be obtained as designed without being affected by the external electromagnetic field.
In the above-mentioned dielectric filter or laminated dielectric filter of the present invention, it is preferred to include capacitor electrodes formed of the transmission lines opposing the resonator electrodes on the outermost sides and connect the capacitor electrodes to the side electrodes to form input/output terminals.
In the above-mentioned dielectric filter or laminated dielectric filter of the present invention, it is preferable that the capacitor electrodes are formed of the transmission lines opposing open ends of the resonator electrodes and are grounded.
According to this configuration, between the open ends of the resonator electrodes and the capacitor electrodes opposing them, loading capacitors as components of the bandpass filter can be formed.
Further, it is preferred to use the filter, dielectric filter, or laminated dielectric filter of the present invention in an antenna duplexer as one of or both of filters on transmission and reception sides.
According to this configuration, a conventional coaxial resonator with a high space factor, which has been used in an antenna duplexer, can be omitted. Therefore, the size of the antenna duplexer can be reduced considerably.
It also is preferred to use the filter, dielectric filter, or laminated dielectric filter of the present invention in communication equipment.
According to the various embodiments of the invention, desired characteristics can be obtained in communication equipment of limited size. Thus, the filter, dielectric filter, or laminated dielectric filter of the present invention also may contribute to the size reduction of the communication equipment.